Chapter 12 of To Kill a Mockingbird by Harper Lee marks the beginning of the second part of the novel, and it focuses on the trial of Tom Robinson. Of course the author has already carefully established all the major characters, and we pretty much know what kind of people they are and how they will react in most circumstances.
One of the things we know about Atticus Finch is that he stands for what is right. He has declared it a sin to kill a mockingbird, which we understand is a metaphor for anyone who does nothing but try to help people. We know that Atticus advocates compassion for others because he tells Scout, his daughter, that it is important to walk in other people's skin in order to understand them better. We know, according to Miss Maudie, that Atticus is the same in his house as he is on the street, which means he practices what he preaches. We know that Atticus wants his children to learn that true courage comes from within rather from without.
In terms of the trial, we know that know that Atticus was especially asked to take this case because he will actually give Tom Robinson a real defense, but I won't give anything more away about that. Suffice it to say that people who see all people as being equal, as Atticus does, are supportive of him; those who are still stuck in their racial prejudices think Atticus is misguided at best and a betrayer of his race, at worst.
Earlier Miss Maudie also told us that no one runs against Atticus for his position as a state representative because they know he is willing to do what needs to be done on their behalf and they let him do it. (And of course this is a commentary both on Atticus's outstanding character and the weakness/laziness of those he represents.) The cartoon which appears in the Montgomery Advertiser is a reflection and a reiteration of this fact. State government is in session, and Atticus is working to change things for the better in his state. Scout explains the cartoon:
We were surprised one morning to see a cartoon in the Montgomery Advisor above the caption, "Maycomb's Finch." It showed Atticus barefooted and in short pants, chained to a desk: he was diligently writing on a slate while some frivolous-looking girls yelled "Yoo-hoo!" at him.
Scout does not really get the significance of this satirical cartoon, but we are thankful to have her description. Like so many other things in this novel, Scout is the one who tells us things, but it is her older brother Jem who explains them for her (and thus for us). He says:
“That’s a compliment,” explained Jem. “He spends his time doin‘ things that wouldn’t get done if nobody did ’em.”
Jem's assessment is correct. Atticus is working hard to make things better for everyone, despite the fact that others don't care as much as he does or even see the need for the changes he wants to make. He is purposeful and deliberate, and he is not distracted by things that don't matter.
Even though many of his fellow citizens make fun of him and even taunt him in this public way, Atticus is committed to make his world better. We are about to see that play out as he defends a black man who needs someone to believe him and fight for him. Atticus is just the right person for that job, and this cartoon is just a small way to remind us (the readers) of this aspect of Atticus's character as we head into the trial.